Treatment of hygroscopic material

ABSTRACT

An apparatus for providing a water vapor-air mixture for treating a hygroscopic material having a mixing chamber, supply for providing air to the mixing chamber at a temperature in the range of 0° C. to 80° C. and at a pressure in the range of 1 to 3 bar, supply for providing steam to the mixing chamber at a temperature in the range of 100° C. to 25° C. and at a pressure in the range of 1 to 10 bar, the mixing chamber having an outlet in connection with a treatment chamber to provide the treatment chamber with a water vapor-air mixture at a temperature below 200° C. and at a pressure in the range of 1 to 5 bar.

BACKGROUND OF THE INVENTION

This invention relates to the treatment of a hygroscopic material suchas tea or tobacco. Such treatments are carried out, for example, withthe intention of increasing the materials pliability by the introductionof moisture and heat into the material or with the intention ofintroducing cellular expansion. The introduction of pliability isadvantageous since it reduces the material's fragility and the materialbecomes better able to resist mechanical damage in subsequent handling.The introduction of cellular expansion is advantageous for products madefrom the material where a principle judgement criteria is minimisationof the mass of material required to occupy a given volume. The relevancyof the invention can be illustrated by reference to tobacco processing.

It is well known that moisture penetration into the structure of ahygroscopic material requires a heat energy input known as the energy ofmoisture adsorption. This energy may be derived from the surroundingenvironment gradually with time, or more quickly by passing steamthrough the material to provide both heat and moisture.

It is well known that hygroscopic organic materials such as tobacco arethermally sensitive and that their exposure to heat will introducechemical change and related changes in their physical properties. Inparticular heating of the material, while inducing temporary pliabilityto the product while it is at elevated temperature, will also inducechemical change so that when the material cools and loses it's temporarypliability, it's pliability at normal temperature and moisture isactually less that it was prior to the heating operation. Further thehigher the temperature the material is subjected to, the less pliableand more fragile it becomes when it reverts to normal temperatures.

This is illustrated below, which shows the effect of average tobaccotemperatures as it exits from an expansion process and the quantity ofsmall particles in the tobacco after it has been reduced to normaltemperature and moisture by a subsequent drying process.

    ______________________________________                                                         Tobacco Concentration of                                     Tobacco Average temperature                                                                    Small Particles after                                        at Exit from Expansion                                                                         Subsequent Drying Process %                                  Process °C.                                                                             below 1 mm                                                   ______________________________________                                         94              8.0                                                          102              8.5                                                          104              11.4                                                         ______________________________________                                    

The results indicate that as the expansion process average temperatureincreases so does the quantity of small particles in the resultanttobacco product. This increase in small particles will lower theefficiency of the subsequent manufacturing process and increase thewastage of tobacco by increasing the quantity of dust removed.

It is the current expert view that tobacco cellular expansion resultsfrom an increase of water vapour pressure within the cell. One form ofprocess equipment to achieve cellular expansion in this way is given inPatent GB2138666 in which a substantially horizontal vibrating tunnel isused to convey tobacco and steam is emitted from the base to theinterior of the tunnel and passes through the transporting tobacco. Thatpatent indicates average tobacco temperatures of 100.5° C. to 120° C.resulting from the use of steam at 2.5 to 25 bar and at steamtemperature of 126° C. to 400° C.

In this apparatus steam is emitted into the tunnel in comparativelywidely spaced streamlets and in practise the apparatus is operatedtypically with 3 to 7 bar pressure. For a tunnel 2.0 meter long by 0.4meter wide GB2138666 utilises 7 rows of 15 holes per row and 0.8 mmdiameter.

In operation an average product temperature of about 105° C. resultsfrom the use of steam at 5 bar having a temperature of 152° C. Inpractice, however, some particles of tobacco attain close to the steamtemperature ie, 152° C. while other particles experience fewer contactswith the steam streamlets and will only reach lower temperatures.

In consequence the resultant average tobacco temperature of 105° C. ismade up of particles with temperatures below 105° C. and other particleswith temperatures of up to 152° C.

Particles which have not received sufficient heat will experience lowerthan average cellular expansion, while particles which have reachedhigher than average temperatures will have an increased fragility and bemore likely to size degrees during subsequent handling as wasillustrated in the table above.

The disadvantages of GB2138666 are partially alleviated by U.S. Pat. No.5,161,548 which uses steam pressure and a far greater number of steamstreamlets. U.S. Pat. No. 5,161,548 typically uses 5,000 steamstreamlets where GB2138666 would use 105 streamlets. However, in bothcases the treatment gas is steam which has in relation to it's mass alevel of volume, temperature and heat which is determined by it'spressure.

Consequently the use of GB2138666 or U.S. Pat. No. 5,161,548 to give anaverage tobacco temperature of say 70° C. still subjects some of thetobacco particles to steam at 100° C. since this is the lowesttemperature of steam at normal atmospheric pressure.

A further application of this current invention is in conjunction with ametering tube as disclosed in GB1559507. In GB1559507 tobacco is passeddown a substantially vertical metering tube or column. The tube isarranged to have a band of perforations running around it's diameter.Steam is passed through the perforations to heat and moisten the tobaccoflowing through the tube. Process apparatus of this form may be used aspart of a tobacco cellular expansion process or as a conditioningprocess. A common application is to condition rejected cigarettes priorto their entry into a separate machine which recovers tobacco from thecigarettes so that the tobacco can be re-used. It is important that thecigarettes at entry to the reclaim have sufficient moisture content tominimise the tobacco damage occurring during the reclaim operation.

Typically reject cigarettes will have a moisture content of 8 to 14%while the desirable moisture at entry to the reclaim plant is 16 to 18%.Hence there is a requirement to add a controlled amount of water to givea moisture rise of 2 to 10% and also to operate at as lower temperatureas possible in order to minimise temperature induced changes to thetobacco's chemical and physical properties.

As steam flows over the cigarettes it will lose heat and moisture bycondensation which in turn raises the temperature and moisture contentof the cigarette. This process continues until the cigarette reaches thesteam temperature.

As the condensation occurs and removes moisture from the steam, thesteam volume decreases. This means that, considering the metering tubeexample, the cigarettes close to the steam entry perforations must reachclose to the steam temperature before steam can flow past them tocondition other cigarettes.

A frequently met practical consequence is that at the tube dischargecigarettes near the periphery of the tube are hot and have gainedmoisture while those that flowed down the centre of the tube may be cooland have received very little moisture gain.

The moisture gained by these cigarettes in contact with the steam isdependent on their specific heat and initial temperature. This gain canbe calculated to be usually in the range of 2.5 to 5.0% compared to thedesired gain of 2 to 10%. Further, once the cigarettes have left thetube, they will start to experience evaporative cooling and the moisturecontent of the cigarette will reduce. A typical evaporative cooling lossis about 1.0%.

For a cigarette input moisture to the tube of 8% the expected moistureat the entry to cigarette reclaim becomes 9.5 to 12% or for tube entrymoisture of 14% the reclaim entry expected moisture becomes 15.5 to 18%compared to the desired 16 to 18%. Consequently a large proportion ofthe input cigarettes are at risk of being conditioned to below thedesired moisture, and those cigarettes which have been conditioned havealso been subjected to detrimental temperatures.

The moisture gain of tobacco from steam is limited by temperaturebalance and ceases when the tobacco and steam reach the sametemperature. The moisture gain of tobacco from a gas which is a mixtureof air and water vapor is limited by vapor pressure balance. Moisturewill continue to transfer from the air to the tobacco until the vaporpressure of water in the tobacco equals the vapor pressure of the waterair mixture. This is illustrated by the fact that tobacco left in anenvironment of 22° C. 75% relative humidity can eventually reachequilibrium moistures of 25 to 30% irrespective of their startingmoisture.

Consequently if a conditioning metering tube is supplied with a gas madeup of a mixture of air and water vapor greater tobacco moistureincreased can be obtained at lower gas and tobacco temperatures thenwould result from the use of steam.

The vapor pressure, temperature, volume and heat content of the gas canbe pre-determined by mixing controllable quantities of air, steam waterspray in a mixing chamber which can contain additional heating elements.That prepared gas mixture is then supplied to a suitable process machinefor application to the tobacco.

It is now being realised, however, that subjecting certain types oftobacco to temperatures in excess of 100° C. or more can damage thetobacco structure, natural soluble or volatile organic compounds can bedriven off, and, in general, the character of the tobacco can bediminished.

One method of treating tobacco which does not involve high temperaturescomprises the intensive soaking of tobacco rib material in water. Thisis a well accepted method of treating tobacco. Heat is absorbed eithersimultaneously or subsequently to enable the ribs to expand.

Whilst this treatment is relatively gentle, a secondary treatmentcomprising rapid drying of the exterior whilst retaining the moisturewithin the rib is also required. A further problem encountered withwater soaking is that the resulting product can be objectively stickysince resinous water extracted solids tend to remain on the surface ofthe tobacco. This sort of treatment can also damage the tobaccostructure, can remove water soluble compounds and the character of thetobacco can be diminished.

SUMMARY OF THE INVENTION

The present invention is based upon the finding that to be suitablytreated by moisture, a hygroscopic material such as tobacco does notalways need to be heated at temperatures in excess of 100° C. nor besoaked in water or water solutions to improve its characteristics forfurther processing.

According to the invention there is provided a process for treating ahygroscopic material comprising contacting the hygroscopic material witha mixture of air and water vapor at a temperature of less than 200° C.,preferably approximately 100° C. or less than 100° C., preferably in therange of 50°-200° C. and at a pressure of 1 to 1.5 bar to increase thetemperature of the hygroscopic material without reducing its watercontent. This has the effect of increasing the specific volume of thematerial without it being subjected to damaging high temperatures ordrying out.

Preferably the gas mixture is prepared in an area remote from where thehygroscopic material contacts the vapor/air mixture. This enables thewater vapor-air mixture to be evenly heated and to have a uniformpredetermined moisture content before application to the hygroscopicmaterial. In order to compensate for the lower temperatures used, theflow rate of the mixture is greater than in prior art devices and/or theconditioning times are increased. The mixture is preferably produced bya mixing mass of air having a moisture content determined by ambientconditions at a first temperature in the range of 0° to 50° C. and at afirst pressure in the range of 1 to 3 bar with a mass of steam at asecond temperature in the range of 100° to 250° C. and at a secondpressure in the range of 1 to 10 bar. Further water in the form of anatomised spray may be introduced into the mixture to increase the degreeof saturation and additional heat energy added by suitable heaters.

This enables the gas mixture, volume, total water content, total heatcontent and temperature to be adjusted substantially independently ofthe gas mixture pressure.

According to a further aspect of the invention there is provided anapparatus for providing a water vapor-air mixture for treating ahygroscopic material comprising a mixing chamber, means for providingair to the mixing chamber at a temperature in the range of 0° to 80° C.and at a pressure in the range of 1 to 3 bar, means for providing steamto the mixing chamber at a temperature in the range of 100° to 250° C.and at a pressure in the range of 1 to 10 bar, the mixing chamber havingan outlet in connection with a treatment chamber to provide thetreatment chamber with a water vapor-air mixture at a temperature below200° C. and at a pressure in the range of 1 to 1.5 bar. The mixingchamber has an outlet which is connected to a treatment chamberincluding means to convey the hygroscopic material and the mixingchamber can provide the treatment chambers with a water vapor-airmixture at a temperature below 200° C. preferably below 100° C.,preferably 50°-200° C. and at a pressure in the range of 1 to 1.5 bar.

Preferably the gas mixing chamber further comprises a water inlet meansto enable water to be sprayed into the mixing chamber. Preferably theconveying means comprises a conveyor which can convey the hygroscopicmaterial through the treatment chamber so as to expose the hygroscopicmaterial to the water vapor-air mixture.

The invention also provides, according to a further aspect, apparatusfor conditioning a hygroscopic material comprising a treatment chamberin which the hygroscopic material may be treated, and means forproviding the treatment chamber with a water vapor-air mixture at atemperature of less than 200° C. and at a pressure of 1 to 1.5 bar toincrease the temperature of the hygroscopic material without reducingit's water content. Hitherto, the hygroscopic material has been treatedin a treatment chamber and pure steam has been injected into thetreatment chamber to provide the desired pressure, temperature andhumidity.

In accordance with this arrangement of the invention, greater control ofthe air-steam mixture is provided and greater homogeneity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a schematic diagram of an apparatus for conditioning ahygroscopic material.

FIG. 2 is an energy flow diagram, and

FIG. 3 is a graphical representation of possible values for the mixturetemperatures.

DETAILED DESCRIPTION

With reference to FIG. 1. air is introduced into a gas preparationmixing chamber 10 through inlet 18 at a pressure of 1 to 3 bar by meansof a compressor 11 such as an eight stage centrifugal fan. A silencerand filter 12 are fitted on the intake of the fan to reduce noise levelsand to ensure that the air is clean. The compressor 11 is driven by anelectric motor (not shown). The air temperature is measured by a monitor14 whilst the flow rate is measured by a flowmeter 15 which in turn isconnected to a throttle value 16 at the intake of the fan. Date from thesensors 14, 15 relayed to a process control and display unit 17. Theconnections from the various sensors to the process control and displayunit are indicated by dashed lines.

Steam is ideally supplied from two sources, and in this case from afactory steam supply 20 via a pressure reducing valve 21 and from asteam producing unit 22 via a pressure reducing valve 23 and a controlvalve 24. The steam inlet pipes from the separate steam sources 20, 22meet at junction 19. The steam pressure is monitored by pressure gauges25 and 26 and the steam temperature by temperature gauge 27.

The conduit 28 leading from the junction 19 is provided with a globevalve 29, a pressure reducing valve 30 a pressure gauge 31 and a controlvalve 32 and is connected to chamber 10 where a distributing probe 34inside chamber 10 provides an arrangement of steam outlets to ensurethorough mixing of the steam with the air.

A further conduit 46 transfers the prepared and mixed gas to a processmachine 50 as described in, for example, GB1955907, GB2138666, U.S. Pat.No. 5,161,548.

In use, steam is introduced into the mixing chamber 10 at a temperaturein the range of 100° C. to 300° C., typically 250° C., under pressuresof 1 to 10 bar, typically 3 bar. Air is supplied at atmospherictemperature in the range of 0° to 80° C. and is pressurised up to 3 barso that the mass of steam to air is in the range of 1:1 to 10:1(steam:air), preferably in the range of 1:1 to 5:1. This results in awater vapor-air mixture at temperatures in the range of 50° C. to 200°C., i.e., a water vapor-air mixture which may be controlled to form asuperheated, a supersaturated or a saturated mixture. If the airentering the mixing chamber 10 is hot (80° C.+), due to high ambienttemperature (up to 50° C.) combined with the temperature increasethrough compressor 11 (approx. 50° C.), then steam and water or wateronly from a factory supply 39, suitably filtered to remove unwantedcompounds may be introduced into the chamber by an atomising inlet 43,the supply of water being monitored by a flow meter 44 and a pressuregauge 45.

The resulting water vapor-air mixture is then fed via conduit 46 to thetreatment chamber 50 at a pressure slightly above atmospheric.

The mixture pressure should be sufficiently above atmospheric to ensurethat in the treatment chamber 50, the vapor-air mixture can percolatethrough the material being treated.

The following example is now given with reference to FIG. 2 which is anenthalpy flow diagram where a mass of air A and a mass of steam Scombine in chamber C to produce a water vapor-air mixture M.

In the following equations:

m_(a) =mass of air (kg)

m_(s) =mass of steam (kg)

m_(v) =mass of water vapor-air (kg)

h_(a) =enthalpy of air at inlet temperature (kJ/kg)

h_(s) =enthalpy of steam at inlet temperature (kJ/kg)

h_(a2) =enthalpy of air at final temperature (kJ/kg)

h_(v) =enthalpy of vapor-air at final temperature (kJ/kg)

T₁ =temperature of air on entry to mixing chamber 10

T₂ =temperature of steam on entry to mixing chamber

ω=m_(s) /m_(a) =specific humidity

h_(a) =c_(pa) ΔT (kJ/kg)

c_(pa) =heat capacity of air (kJ/kgK)

ΔT=temperature change (k)

h_(v) =h_(g) +C_(ps) ΔT (kJ/kg)

c_(ps) =1.86 (kJ/kgK)

h_(g) =saturated vapor-air enthalpy (kJ/kg) (obtained from tables atP=P_(s))

T_(g) =saturation temperature (°C.) (obtained from tables P=P_(s))

T₃ =final temperature of mix (°C.) P=mixture pressure (bar)

P_(s) =pressure due to vapor after mixing (bar)

Enthalpy values determined from 0° C. datum.

Using the following steady state flow equation:

    m.sub.a h.sub.a1 +m.sub.s h.sub.s =m.sub.a h.sub.a2 +m.sub.v h.sub.v

As the mass of water is constant (even though it may be in a differentphase),

    m.sub.v =m.sub.s

Therefore ##EQU1##

EXAMPLE 1

Assuming:

Intake air is dry, at 1.013 bar pressure and T₁ at 20° C.

Input steam is saturated at 3 bar pressure and T₂ at 133.5° C.

Mixture pressure=1.013 bar at tobacco h_(s) =2725 kJ/kg ##EQU2##

From tables, T₃ =87° C.; h_(g) =2655 kJ/kg ##EQU3##

Thus the mixture temperature is 87.9° C.

EXAMPLE 2

Assuming:

Intake air is dry, at 1 bar pressure and T₁ is 20° C.

Input steam is saturated at 1.013 bar pressure and T₂ is 100° C.

h_(s) =2675.8 KJ/Kg

T₃ =70.8° C.

Thus the mixture temperature is 70.8° C.

For the same degrees of saturation, temperature and pressure of theinput steam and air, by adjusting the ratios of the mass of steam to themass of air (steam:air) as follows,

then 1.5:1 results in a mixture temperature of T₃ =77.7° C.

and 5:1 results in a mixture temperature of T₃ =91.5° C.

FIG. 3 shows the range of possible values for the mixture temperature T₃assuming dry intake air at temperatures 20°, 50°, 70° and 90° C.

Whilst the invention has been described in relation to a tobaccoprocessing apparatus, the mixing chamber may be fitted to new plant ormay be fitted to existing machinery where appropriate steam and waterexists.

We claim:
 1. An apparatus for providing a water vapour-air mixture for treating a hygroscopic material comprisinga mixing chamber (10), means for providing air (11) to the mixing chamber (10) at a temperature in the range of 0° to 80° C. and at a pressure in the range of 1 to 3 bar, means for providing steam (20,22) to the mixing chamber at a temperature in the range of 100° to 250° C. and at a pressure in the range of 1 to 10 bar, the mixing chamber having an outlet (46) in connection with a treatment chamber (50) to provide the treatment chamber (50) with a water vapour-air mixture at a temperature below 200° C. and at a pressure in the range of 1 to 1.5 bar.
 2. An apparatus as claimed in claim 1 in which said means for providing air (11) to the mixing chamber (10) is adapted to provide the air at a temperature in the range of 0° to 50° C.
 3. An apparatus as claimed in claim 1 in which said means for providing air (11) to the mixing chamber (10) is adapted to provide the air at a temperature in the range of 30° to 50° C.
 4. An apparatus as claimed in claim 1 in which said means for providing air (11) to the mixing chamber (10) is adapted to provide the air at a temperature in the range of ambient plus 5° C. to ambient plus 30° C.
 5. An apparatus as claimed in claim 1 in which said means for providing air (11) to the mixing chamber (10) is adapted to provide air at a pressure in the range of 1 to 1.5 bar.
 6. An apparatus as claimed in claim 1 in which said means for providing steam (20,22) to the mixing chamber (10) is adapted to provide said steam at a pressure in the range 1 to 4 bar.
 7. An apparatus as claimed in claim 1 in which said means for providing steam (20,22) to the mixing chamber (10) is adapted to provide said steam at a pressure in the range 1 to 3 bar.
 8. An apparatus as claimed in claim 1 in which the temperature of the water vapour-air mixture provided is approximately 100° C.
 9. An apparatus as claimed in claim 1 in which the pressure of the water vapour-air mixture is in the range 1 to 1.1 bar.
 10. An apparatus as claimed in claim 1 in which the pressure of the water vapour-air mixture is in the range 1 to 1.05 bar.
 11. An apparatus as claimed in claim 1 in which the temperature of the water vapour-air mixture is below 100° C.
 12. An apparatus as claimed in claim 1 in which the temperature of the water vapour-air mixture is in the range 50°-200° C.
 13. An apparatus as claimed in claim 1 further comprising a water inlet means (42) to enable water to be sprayed into the mixing chamber.
 14. An apparatus as claimed in claim 1 further comprising means to convey the hygroscopic material through the treatment chamber so as to expose the hygroscopic material to the water vapour-air mixture for a period of time.
 15. A process for conditioning a hygroscopic material comprising:providing air to a mixing chamber at a temperature in the range of 0° C. to 80° C. and at a pressure in the range of 1 to 3 bar; providing steam to said mixing chamber at a temperature in the range of 100° C. to 250° C. and at a pressure in the range 1 to 10 bar; providing a water vapour-air mixture from said mixing chamber at a temperature below 200° C. at a pressure in the range 1 to 1.5 bar to contact and treat said hygroscopic material whereby the temperature of the hygroscopic material is increased without reducing its water content.
 16. A process as claimed in claim 15 in which the water vapour-air mixture is at a temperature of approximately 100° C.
 17. A process as claimed in claim 15, in which the water vapour-air mixture is at a temperature of less than 100° C.
 18. A process as claimed in claim 15 in which the water vapour-air mixture is at a temperature in a range 50°-200° C.
 19. A process as claimed in claim 15 wherein the water vapour-air mixture is produced in an area remote from an area where it contacts the hygroscopic material.
 20. A process as claimed in claim 15 wherein the steam is saturated.
 21. A process as claimed in claim 15 wherein the steam is super saturated.
 22. A process as claimed in claim 15 wherein the temperature of the hygroscopic material after contacting the air and water vapour mixture does not exceed 100° C.
 23. A process as claimed in claim 15 wherein the hygroscopic material is tobacco.
 24. Apparatus for conditioning a hygroscopic material comprising a treatment chamber in which the hygroscopic material may be treated, and means for providing the treatment chamber with a water vapour-air mixture at a temperature of less than 200° C. and at a pressure of 1 to 1.5 bar to increase the temperature of the hygroscopic material without reducing it's water content. 